Starting with the Diablo cycle, OpenStack core projects use a branching model close to the NVIE model that ensures an almost-always-open master while still allowing to freeze features and select bugfixes in a release branch (milestone-proposed):

At some point before the release of a milestone (called the "branch point"), the milestone-proposed branch gets created out of the master branch. The master branch continues towards the development of the next milestone. The milestone-proposed branches gets testing and backported bugfixes (that must land in master first), until the release is tagged.

Version numbers

Since we produce artifacts that are leading up to a release and drive a lot of automation, we produce automatic version numbers. Prior to Grizzly, we used a different versioning scheme, which remains documented at OldVersionNumbers